1. Field of the Invention
The present invention relates to an enameled wire for use in an electric motor, etc.
2. Prior Art
In order to improve the energy efficiency of an electromechanical device having an electric motor, the variable speed control using an inverter is becoming popular. The inverter is usually driven at a frequency in a range from about 2 KHz to several tens of KHz and generates a surge voltage for every, for example, PWM pulse. As is well known, the surge voltage, which is higher than an output voltage of the inverter, depends upon the conditions of the environmental electrical system, for example, cable length and capacitance of the system. If any sine waveform of the surge voltage is sharp, there is a tendency that partial discharges of enameled wire of an electromagnetic device such as a motor occurs. With such partial discharge of the enameled wire, insulating characteristics of the enamel film degrade with increasing speed under the complicated influence of the locally increased temperature of the wire's enamel film and ozone generated by the discharge, resulting in shortening of the life of the electromagnetic equipment.
The durability of enameled wire against surge voltage may be improved to some extent by increasing thickness of the enamel coating film of the enameled wire and/or increasing an amount of resin impregnated in the winding of the motor. In such a case, however, the energy efficiency of the motor is lowered by increased space factor and the cost of the motor is increased. In addition to these problems, there may be cases where desired reliability of the motor can not be obtained. In order to solve these problems, an enamel coating film of an enameled wire has to have superior characteristics against surge voltage of the inverter.
Development of enamel coating film having superior characteristics against inverter surge has been promoted recently. For example, JPH11-126517 of Essex Group Inc., discloses an enameled wire coated with a coating layer containing 10-50 weight % of silica or chromium oxide particles. A catalog of Phelps Dodge, Inc., discloses an enameled wire having a three-layer structure including an intermediate layer called “Quantum Shield Layer” in which metal oxide is mixed.
Further, JP2000-331539 and JP2001-307557, both of which are assigned to Hitachi Cable Ltd., and a technical report of the same company reported in the National Conference of the Electric Engineers of Japan (5-004), 2001, disclose enameled wires each having an enamel coating film in which 30-100 weight parts of fine particles of metal oxide or silica or 3-100 weight parts of sol compound thereof is mixed are disclosed.
As mentioned above, in order to improve the durability of the enameled wire against surge voltage of an inverter of a device using the enameled wire, the enameled wire using an enamel coating film containing inorganic filler material has been developed. In addition, the enamel coating film having a double layer and the enamel coating film having a triple layer structure, each of which contains fine particles of metal oxide or silica as the inorganic filler material, have been proposed. In either of these proposals, the desired characteristics of the enameled wire can not be obtained unless an amount of the inorganic filler material is 30 weight parts or more to 100 weight parts of resin.